Passive emergency feedwater system

ABSTRACT

A power module assembly may include a reactor vessel containing a primary coolant and one or more inlets configured to draw a secondary coolant from the containment cooling pool in response to a loss of power and/or a loss of coolant. One or more outlets may be submerged in the containment cooling pool and may be configured to vent the secondary coolant into the containment cooling pool. A heat exchanger may be configured to remove heat from the primary coolant, wherein the heat may be removed by circulating the secondary coolant from the containment cooling pool through the heat exchanger via natural circulation.

This application is a continuation of U.S. application Ser. No.13/425,776 filed on Mar. 21, 2012, which is a continuation-in-part ofU.S. application Ser. No. 12/121,733 filed on May 15, 2008, now issuedU.S. Pat. No. 8,170,173, issued May 1, 2012, which claims priority toprovisional application U.S. application Ser. No. 60/988,348, filed Nov.15, 2007, which are all herein incorporated by reference in theirentireties.

TECHNICAL FIELD

The invention relates to a cooling system for a nuclear reactor.

BACKGROUND

In nuclear reactors designed with passive operating systems, the laws ofphysics are employed to ensure that safe operation of the nuclearreactor is maintained during normal operation or even in an emergencycondition without operator intervention or supervision, at least forsome predefined period of time. A Multi-Application Small Light WaterReactor project conducted with the assistance of the Idaho NationalEngineering and Environmental Laboratory, NEXANT and the NuclearEngineering Department of Oregon State University sought to develop asafe and economical natural light water reactor. FIG. 1 illustrates anuclear reactor design 5 that resulted from this project.

The nuclear reactor design 5 includes a reactor core 6 surrounded by areactor vessel 2. Water 10 in the reactor vessel 2 surrounds the reactorcore 6. The reactor core 6 is further located in a shroud 22 whichsurround the reactor core 6 about its sides. When the water 10 is heatedby the reactor core 6 as a result of fission events, the water 10 isdirected from the shroud 22 and out of a riser 24. This results infurther water 10 being drawn into and heated by the reactor core 6 whichdraws yet more water 10 into the shroud 22. The water 10 that emergesfrom the riser 24 is cooled down and directed towards the annulus 23 andthen returns to the bottom of the reactor vessel 2 through naturalcirculation. Pressurized steam 11 is produced in the reactor vessel 2 asthe water 10 is heated.

A heat exchanger 35 circulates feedwater and steam in a secondarycooling system 30 in order to generate electricity with a turbine 32 andgenerator 34. The feedwater passes through the heat exchanger 35 andbecomes super heated steam. The secondary cooling system 30 includes acondenser 36 and feedwater pump 38. The steam and feedwater in thesecondary cooling system 30 are isolated from the water 10 in thereactor vessel 2, such that they are not allowed to mix or come intodirect contact with each other.

The reactor vessel 2 is surrounded by a containment vessel 4. Thecontainment vessel 4 is placed in a pool of water 16. The pool of water16 and the containment vessel 4 are below ground 9 in a reactor bay 7.The containment vessel 4 is designed so that water or steam from thereactor vessel 2 is not allowed to escape into the pool of water 16 orthe surrounding environment. A steam valve 8 is provided to vent steam11 from the reactor vessel 2 into an upper half 14 of the containmentvessel 4. A submerged blowdown valve 18 is provided to release the water10 into suppression pool 12 containing sub-cooled water.

During a loss of feedwater flow, the nuclear reactor 5 is designed torespond by scramming the reactor core 6, flooding the containment vessel4 or depressurizing the reactor vessel 2. The latter two of theseresponses result in the nuclear reactor 5 being shut down and unable togenerate electricity for an extended period of time.

The present invention addresses these and other problems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a nuclear power system known in the art.

FIG. 2 illustrates a novel power module assembly including an secondarycooling system.

FIG. 3 illustrates an embodiment of a novel emergency cooling system.

FIG. 4 illustrates a novel method of cooling a nuclear reactor.

SUMMARY OF THE INVENTION

A power module assembly may include a reactor vessel containing aprimary coolant and one or more inlets configured to draw a secondarycoolant from the containment cooling pool in response to a loss of powerand/or a loss of coolant. One or more outlets may be submerged in thecontainment cooling pool and may be configured to vent the secondarycoolant into the containment cooling pool. A heat exchanger may beconfigured to remove heat from the primary coolant, wherein the heat maybe removed by circulating the secondary coolant from the containmentcooling pool through the heat exchanger via natural circulation.

A power module assembly may include a reactor vessel containing aprimary coolant, means for removing heat from the primary coolant, meansfor drawing emergency feedwater from a containment cooling pool inresponse to a loss of power and/or a loss of coolant, and means forventing the emergency feedwater into the containment cooling pool. Themeans for venting may be submerged in the containment cooling pool. Thepower module assembly may further include means for circulating theemergency feedwater from the containment cooling pool through the meansfor removing heat and back to the containment cooling pool, wherein theemergency feedwater is circulated through natural circulation.

A method may include removing heat from a primary coolant containedwithin a reactor vessel, drawing emergency feedwater into one or moreinlets from a containment cooling pool in response to a loss of powerand/or a loss of coolant, and venting the emergency feedwater from oneor more vents into the containment cooling pool, wherein the one or morevents are submerged in the containment cooling pool. The method mayfurther include circulating the emergency feedwater from the containmentcooling pool through the reactor vessel and back to the containmentcooling pool, wherein the emergency feedwater is circulated throughnatural circulation.

Description of Example Embodiments

Conventional nuclear facilities are expensive to license and build, withsignificant upfront investment costs and delayed return of profits. Inaddition to energy cost considerations, efficiency requirements, andreliability concerns, today's nuclear reactor designs must also takeinto account issues of nuclear proliferation, terrorist activities, anda heightened awareness of environmental stewardship.

Developing countries that could otherwise greatly benefit from nuclearpower are frequently left to resort to other energy sources such ascoal, gas or hydroelectric power generators that produce significantamounts of pollution or have other detrimental environmental impact.These developing countries may not have the technological or naturalresources that enable them to build a nuclear power plant. Countriesthat have already developed nuclear power may be hesitant to introducethese technologies into the developing countries out of concern of theloss of control of the nuclear materials or technology.

Passively safe nuclear power systems help address some of theseconcerns. Further system improvements and innovative designs areexpected to usher in a new era of nuclear power as a globally viableprimary energy source.

In the Multi-Application Small Light Water Reactor (MASLWR) design, aloss of feedwater flow requires actuation of the long-term cooling modeof operation to provide for core cooling. For example, scramming thereactor core, flooding the containment vessel or depressurizing thereactor vessel. The latter two of these responses result in the nuclearreactor being shut down and unable to generate electricity for anextended period of time. Furthermore, no provision was provided in theMASLWR design for decay heat removal during a loss of site power.

FIG. 2 illustrates a novel power module assembly 25 including asecondary cooling system 50. The power module assembly 25 includes aninternally dry containment vessel 44. The containment vessel 44 iscylindrical in shape, and has spherical upper and lower ends. The entirepower module assembly 25 may be submerged in a containment cooling pool46 which serves as an ultimate heat sink. The containment vessel 44 maybe welded or otherwise sealed to the environment, such that liquids andgas do not escape from, or enter, the power module assembly 25. Thecontainment vessel 44 may be bottom supported, top supported orsupported to about its center. Supporting the containment vessel 44 atthe top may facilitate maintenance and removal of the power moduleassembly 25 from the containment cooling pool 46.

A reactor vessel 42 is located or mounted inside the containment vessel44. An inner surface of the reactor vessel 42 may be exposed to a wetenvironment including a primary coolant 100 or liquid, such as water,and an outer surface may be exposed to a dry environment such as air.The reactor vessel 42 may be made of stainless steel or carbon steel,may include cladding, and may be supported within the containment vessel44.

The power module assembly 25 may be sized so that it can be transportedon a rail car. For example, the containment vessel 44 may be constructedto be approximately 4.3 meters in diameter and 17.7 meters in height(length). By completely sealing the containment vessel 44, access to thereactor core 6 may be restricted. Any unauthorized access or tamperingmay be monitored. Furthermore, the subterranean profile of a nuclearpower system makes it less visible and easier to conceal. Thecontainment cooling pool 46 may be covered with a protective shield (notshown) to further isolate the power module assembly 25 from outsidethreats or airborne objects such as planes or missiles.

The containment vessel 44 encapsulates and, in some conditions, coolsthe reactor core 6. It is relatively small, has a high strength and maybe capable of withstanding six or seven times the pressure ofconventional containment designs in part due to its smaller overalldimensions. Given a break in the primary cooling system of the powermodule assembly 25 no fission products are released into theenvironment. The primary coolant 100 remains entirely contained in thecontainment vessel 44.

The reactor core 6 is illustrated as being submerged or immersed in aprimary coolant 100, such as water. The reactor vessel 42 houses theprimary coolant 100 and the reactor core 6. A shroud 22 surrounds thereactor core 6 about its sides and serves to direct the primary coolant100 up through an annulus 23 and out a riser 24 located in the upperhalf of the reactor vessel 42 as a result of natural circulation of theprimary coolant 100. In one embodiment, the reactor vessel 42 isapproximately 2.7 meters in diameter and includes an overall height(length) of 13.7 meters. The reactor vessel 42 may include apredominately cylindrical shape with spherical upper and lower ends. Thereactor vessel 42 is normally at operating pressure and temperature.

In one embodiment, the containment vessel 44 is internally dry and mayoperate at atmospheric pressure with wall temperatures at or near thetemperature of the containment cooling pool 46. The containment vessel44 may be internally dry during normal operation of the power moduleassembly 25. In some examples, the containment vessel may remaininternally dry after an emergency operating condition is detected and/orafter an emergency operation is initiated in response to the emergencyoperating condition.

The emergency operating condition may include a loss of secondarycoolant flow, a loss of on-site power, and/or a loss of off-site power.For example, a loss of power condition may result when the electricitygrid providing power to the power module assembly 25 and/or auxiliarysystems fails to provide power, or fails to provide sufficient power tooperate one or more electrical devices or systems. The electricity gridmay comprise an external power grid operated by a power company, or aninternal grid developed primarily for plant operation. In still otherexamples, a loss of power condition may result when an on-site generatorruns out of fuel and/or When an on-site power cell, fuel cell, battery,or other type of electrical storage device runs out of power.

During normal operation, thermal energy from the fission events in thereactor core 6 causes the primary coolant 100 to heat. As the primarycoolant 100 heats up, it becomes less dense and tends to rise up throughthe riser 24. As the primary coolant 100 cools down, it becomesrelatively denser than the heated coolant and is circulated around theoutside of the annulus 23, down to the bottom of the reactor vessel 42and up through the shroud 22 to once again be heated by the reactor core6. This natural circulation causes the primary coolant 100 to cyclethrough the reactor core 6, transferring heat to a secondary coolingsystem 50 to generate electricity at a generator, such as generator 34of FIG. 1 .

The secondary cooling system 50 comprises an inlet line 53 configured todeliver a secondary coolant to a heat exchanger 55 that operates as aheat sink for the power module assembly 25. An outlet line 52 isconnected to the heat exchanger 55 and is configured to remove heat fromthe reactor core 6 by circulating the secondary coolant through theprimary coolant contained in the reactor vessel 42. The inlet line 53and outlet line 52 therefore serve as a means of delivery of thesecondary coolant for the secondary cooling system 50. Relatively coolsecondary coolant is transported to the heat exchanger 55 via the inletline 53, whereas as relatively hot or superheated coolant is transportedfrom the heat exchanger 55 to the generator via the outlet line 52.

The secondary cooling system 50 further comprises one or more inletports 54 adapted to supply emergency feedwater to the inlet line 53. Inone embodiment, the emergency feedwater supply is contained in thecontainment cooling pool 46. The containment cooling pool 46 may containwater or some other liquid coolant. One or more outlet valves 58 may beconnected to the outlet line 52 and configured to vent the emergencyfeedwater after it has circulated through the heal exchanger 55. One ormore valves, such as valve 56 and/or valve 69 may be provided betweenthe inlet port 54 and the inlet line 53 to direct the flow of emergencyfeedwater or secondary coolant to the inlet line 53. Valve 56 may belocated at an elevation that is above the containment vessel 44 suchthat the emergency feedwater may be added to the inlet line 53 at aposition above the containment vessel 44. Valve 69 may be located at alower elevation such that the emergency feedwater may be added to theinlet line 53 at a point adjacent the heat exchanger 55. In one example,the emergency feedwater may be added directly to the heat exchanger 55after passing through valve 69.

Although FIG. 2 illustrates both valve 56 and valve 69, some examplesonly include one or the other of the valves 56, 69. Still other examplesmay include both valves 56, 59 that may be configured to operateconcurrently and/or as backup systems for each other. The inlet port 54may include an inlet screen to filter contaminants in the containmentcooling pool 46. A check valve 66 may be configured to limit fluid flowin one direction.

The secondary cooling system 50 may be configured to circulate theemergency feedwater through the heat exchanger 55 by naturalcirculation. The natural circulation may be accomplished due to atemperature difference between the liquid in the containment coolingpool 46 and the primary coolant. The liquid, or emergency feedwater,undergoes a temperature change as it passes through the heat exchanger55. The natural circulation may further be accomplished or augmented asa result of the temperature change of the emergency feedwater and anelevation difference of the inlet ports 54 and the outlet valves 58. Inone embodiment, the one or more inlet ports 54 are located near thebottom of the containment cooling pool 46.

As previously described, the inlet line 53 may be connected to afeedwater pump, such as pump 38 of FIG. 1 , and the outlet line 52 maybe connected to a steam turbine, such as turbine 32 of FIG. 1 . Thesecondary cooling system 50 is able to provide natural circulation ofthe emergency feedwater without the assistance of a feedwater pump orexternal power source.

During a loss of power scenario, an external source of feedwaterprovided through inlet line 53 may be made temporarily or permanentlyunavailable. For example, feedwater pump 38 (FIG. 1 ) may cease tofunction as a result of the loss of on-site power. As discussed above,emergency feedwater obtained from the containment cooling pool 46 maynevertheless be provided to the heat exchanger via natural circulationprovided by the secondary cooling system 50. The secondary coolantand/or emergency feedwater remain physically and/or chemically separatedfrom the primary coolant both prior to and during any emergencyoperating condition. Whereas heat contained within the primary coolantmay be transferred to the secondary coolant and/or emergency feedwaterpassing through the heat exchanger, the coolants themselves remainisolated from each other such that they are not allowed to intermix.

During a loss of feedwater flow scenario, a reactor scram and turbinetrip are initiated by an actuation signal. As steam pressure in thesecondary cooling system 50 increases, the one or more outlet valves 58are opened in a staged manner to depressurize the steam generator. Theone or more outlet valves 58 may include redundant fail safe vent valvesthat discharge the secondary coolant to a set of submerged spargernozzles located below the surface of the containment cooling pool 46.After a preset time delay from actuation of the outlet valves 58, asecond set of valves associated with the one or more inlet ports 54 maybe opened in such a way as to align the feedwater supply piping with thebottom of the containment cooling pool 46.

As the steam generator depressurizes, a natural circulation flowpath isestablished from the inlet ports 54 through the heat exchanger 55 andout the outlet valves 58. Liquid from the containment cooling pool 46provides a makeup source of emergency feedwater. Steam produced in theheat exchanger 55 and released through the outlet valves 58 is condensedin the containment cooling pool 46. The outlet valves 58 may bepositioned just below the surface of the containment cooling pool 46.

The secondary cooling system 50 may be configured to remove the liquidfrom the containment cooling pool 46 during one or more types ofemergency operation. The emergency operation may include a loss ofcoolant accident, a loss of secondary coolant flow, a loss of on-sitepower, among others.

FIG. 3 illustrates an embodiment of a novel cooling system 60. Thecooling system may be configured to operate with a nuclear reactor suchas the power module assembly 25 of FIG. 2 . The cooling system 60comprises inlet line 53 and outlet line 52 configured to remove heatfrom a reactor core by circulating a coolant through heat exchanger 55.Relatively cool coolant is transported to the heat exchanger 55 via theinlet line 52, whereas as relatively hot or superheated coolant istransported from the heat exchanger 55 to the generator via the outletline 52.

The cooling system 60 further comprises one or more inlet ports 54adapted to supply emergency feedwater to the inlet line 53. In oneembodiment, the emergency feedwater supply is contained in a coolingpool 46 (FIG. 2 ), and comprises water or some other coolant. A coolingpipe 57 connecting the inlet port 54 to the inlet line 53 may beinsulated to reduce an amount of heating of the emergency feedwatertraveling through the cooling pipe 57 that might otherwise occur due toa temperature difference of the coolant located at the top and bottom ofthe cooling pool. One or more outlet pots 58 may be connected to theoutlet line 52 to vent the emergency feedwater after it has circulatedthrough the heat exchanger 55. The emergency cooling system 60 furthercomprises one or more accumulator tanks 70 configured to inject coolantinto the inlet line 53 when a loss of feedwater flow is detected. Theone or more accumulator tanks 70 provide coolant to the heat exchanger55 until natural circulation of the emergency feedwater is establishedvia the inlet ports 54 and outlet ports 58.

The one or more accumulator tanks 70 may be partially filled with water.The accumulator tanks 70 may be pressurized with a non-condensable gas,such as nitrogen. In one embodiment, a bladder 71 is provided with, orin, the accumulator tank 70 to prevent the release of thenon-condensable gas (e.g. nitrogen) into the heat exchanger 55. During aloss of feedwater flow scenario, the accumulator tanks 70 inject thewater into the inlet line 53. The injection of water serves to subcoolthe water in the reactor vessel while natural circulation is establishedin the cooling system.

Operation

Example operations of various embodiments is now provided, makingreference to the secondary cooling system 50 of FIG. 2 and the emergencycooling system 60 of FIG. 3 . The reactor core 6 undergoes a hotshutdown condition with control rods inserted. The shutdown conditionmay result from an emergency operation of the power module assembly 25or the secondary cooling system. A normal steam flow through the outletline 52 and a feedwater flow through the inlet line 53 are isolated. Theinlet port 54 and outlet valve 58 are opened to the containment coolingpool 46. This creates a natural circulation flow path driven by thecoolant density difference and elevation difference between the coldwater at the inlet port 54 and the outlet valve 58.

Cold water from the containment cooling pool. 46 is drawn into the heatexchanger 5, where it is heated and vented into the containment coolingpool 46. The heat exchanger 55 removes heat from fluid, for example thecoolant 100, in the annulus 23 creating a density difference between thefluid inside the riser 24 and the fluid in the annulus 23. Because thereactor core 6 is located at an elevation below the heat exchanger 55, abuoyancy force is created that drives warm fluid up through the shroud22 and riser 24 and drives cold fluid down through the annulus 23 intothe lower plenum 51. This creates a natural circulation flow through thereactor core 6 that removes the decay heat.

During a loss of main feedwater flow, a low discharge pressure of theLow Main Feedwater Pump 72 or a low water level of the Steam Generator74 may result in a reactor trip. After a time delay from the reactortrip, the main steam vent valves (outlet valves 58) are opened in astaged manner. When a low level of the Accumulator tank 70 and a lowdischarge pressure of the Low Main Feedwater Pump 72 are detected, theMain Feedwater Stop Valve 76 is closed. Then, the inlet valve (inletports 54) is opened in order to align the cooling flow to thecontainment cooling pool 46.

In various embodiments, the same operations as discussed for the aboveloss of main feedwater flow condition may be followed during a stationblackout or loss of power to the on-site facility.

During a loss of coolant accident, a low water level of the LowPressurizer 73, a low pressure of the coolant system, or a high pressureof the containment vessel 44 may result in a reactor trip. The MainSteam Isolation Valve 78 and Main Feedwater Stop Valve 76 are closed. Ifthe steam generator pressure 77 does not increase (e.g. there is nosteam generator tube rupture) and the steam generator tube bandpressures are equal, then the outlet valves 58 may be opened in a stagedmanner. For example, the Main Feedwater Stop Valve 76 is closed when alow level of the Accumulator tank 70 and a low discharge pressure of theLow Main Feedwater Pump 72 are detected. Additionally, the inlet port 54may be opened to align the cooling flow to the containment cooling pool46. The outlet valves 58 may also be opened in a staged manner. Areactor sump valve may be opened when a low differential pressure isdetected between the containment vessel 44 and the reactor vessel 42.

FIG. 4 illustrates a novel method of cooling a nuclear reactor. Atoperation 410, a loss of feedwater condition is detected. The loss offeedwater may be due to a loss of coolant accident, a loss of feedwaterpressure, a failed feedwater pump, or a loss of on-site power, such as astation blackout. At operation 420 a feedwater flow from a secondarycooling system is replaced with an emergency feedwater supply.

At operation 430 the emergency feedwater is circulated through a heatexchanger to remove heat from the nuclear reactor. The emergencyfeedwater is circulated through the heat exchanger through naturalcirculation. The natural circulation is due to a difference intemperature between the emergency feedwater supply and the emergencyfeedwater circulating through the heat exchanger.

In one embodiment, the emergency feedwater supply comprises acontainment cooling pool surrounding the nuclear reactor. At operation440, the emergency feedwater is vented into the containment coolingpool. An elevation difference between an outlet port and an inlet portsubmerged in the containment cooling pool may provide for sustainablenatural circulation of the emergency feedwater for more than three days.Depending on the size of the containment cooling pool, in oneembodiment, the natural circulation may be maintained upwards of 90days.

Loss of feedwater flow and decay heat removal is resolved by the variousembodiments disclosed herein. The novel systems add significantcapability to the MASLWR reactor design by providing a passive means ofcooling the nuclear core after a control rod insertion without the needfor external power. Various embodiments are able to provide emergencyfeedwater to the steam generator in the event of a loss of normalfeedwater flow, and provide reactor core decay heat removal of thereactor core subsequent to a reactor control rod insertion.

The containment cooling pool serves as a source of makeup feedwater andas a heat sink for decay heat removal. Various embodiments are able toremove core decay heat by directing coolant from the large containmentcooling pool through the helical coil heat exchanger tubes located inthe reactor vessel annulus. A natural circulation flow path isestablished as hot water and steam are vented into the containment pooland cold water is drawn into the inlet port.

Various embodiments disclosed herein provide alternate long-term coolingmode of operation and an indefinite heat removal of the reactor corewithout operator action. Very little mass is lost form the containmentpool. Various embodiments can be actuated manually to remove decay heatfor maintenance, and the systems are relatively simple, having few ifany moving parts. Passive systems do not require on-site power tooperate, instead relying on the principles of natural circulation.Furthermore, various embodiments provide for quick restart of a reactormodule, providing less operating downtime and increased protection ofinvestor capital.

Although the embodiments provided herein have primarily described apressurized water reactor, it should be apparent to one skilled in theart that the embodiments may be applied to other types of nuclear powersystems as described or with some obvious modification. For example, theembodiments or variations thereof may also be made operable with aboiling water reactor. A boiling water reactor may require largervessels to produce the same energy output.

Having described and illustrated the principles of the invention in apreferred embodiment thereof, it should be apparent that the inventionmay be modified in arrangement and detail without departing from suchprinciples. We claim all modifications and variation coming within thespirit and scope of the following claims.

The invention claimed is:
 1. A power module assembly comprising: areactor vessel containing a primary coolant; a containment vesselsurrounding the reactor vessel, wherein the containment vessel isimmersed in a containment cooling pool; one or more inlets configured todraw emergency feedwater from the containment cooling pool; one or moreoutlets configured to vent the emergency feedwater into the containmentcooling pool; and a heat exchanger coupled between the one or moreinlets and the one or more outlets, and configured to remove heat fromthe primary coolant by circulating the emergency feedwater, via naturalcirculation, from the containment cooling pool through the heatexchanger and back to the containment cooling pool, without mixing withthe primary coolant, wherein at least one of the inlets is sub merged inthe containment cooling pool and is coupled via a sub merged, upwardlyextending conduit to an input line of the heat exchanger.
 2. The powermodule assembly according to claim 1, wherein: at least one of theoutlets is submerged in the cooling pool above the at least one inletand is coupled to an output line of the heat exchanger.
 3. The powermodule assembly according to claim 1, wherein the natural circulationcomprises a circulation path of the emergency feedwater entering the oneor more inlets, passing through the heat exchanger, exiting the one ormore outlets into the containment cooling pool, and re-entering the oneor more inlets.
 4. The power module assembly according to claim 1,wherein the containment vessel prevents the primary coolant fromescaping into the containment cooling pool.
 5. The power module assemblyaccording to claim 1, wherein the containment vessel forms asubstantially dry containment region around the reactor vessel.
 6. Anemergency feedwater system for a nuclear reactor vessel that contains aprimary coolant, comprising: a heat exchanger configured to circulate asecondary coolant through the reactor vessel to remove heat from theprimary coolant, wherein the reactor vessel is positioned within acontainment vessel; a secondary cooling system configured to circulateemergency feedwater, via natural circulation, from a containment coolingpool in which the containment vessel is immersed, through the heatexchanger, during a loss of power event to remove heat from the primarycoolant, without intermixing the primary coolant with the secondarycoolant in the secondary cooling system, or with the emergencyfeedwater; and an inlet submerged in the containment cooling pool andcoupled via a submerged, upwardly extending conduit to an input line ofthe heat exchanger.
 7. The emergency feedwater system according to claim6, further comprising the containment vessel, and wherein thecontainment vessel encapsulates the nuclear reactor vessel.
 8. Theemergency feedwater system according to claim 7, wherein the containmentvessel forms a dry containment region around the nuclear reactor vessel.9. The emergency feedwater system according to claim 7, wherein thecontainment vessel is configured to prohibit a release of the primarycoolant into the containment cooling pool.
 10. The emergency feedwatersystem according to claim 6, further comprising: an outlet submerged inthe containment cooling pool and coupled to an output line of the heatexchanger.
 11. The emergency feedwater system according to claim 10,including: a first valve selectively coupling the inlet to the inputline; and a second valve selectively coupling the outlet to the outputline.
 12. The emergency feedwater system according to claim 10, whereinthe outlet is located in the containment cooling pool above the inlet.13. The emergency feedwater system according to claim 6, wherein thesecondary cooling system does not include a pump to circulate theemergency feedwater from the containment cooling pool.